This invention relates to a printed circuit or wiring board having a plated outwardly flaring through-hole and a method of making the same.
As will be described later with reference to several figures of the accompanying drawings, a printed wiring board is manufactured by applying a photoresist film on each principal surface of the board, exposing the photoresist film to light through a mask having a positive or a negative mask pattern, removing the photoresist film to uncover a desired area of the principal surface, and depositing a conductive pattern on the desired area. A printed wiring board generally has a plurality of through-holes. The conductive pattern is formed also in the through-holes to provide supports for circuit elements to be mounted on the board and electrical connections thereto and between the conductive pattern portions formed on both principal surfaces. In order to insure the supports and connections, edges or borders of the conductive pattern have been extended onto those portions of the principal surface which surround the through-hole openings. Due to the pattern borders, it has been unavoidable that a restriction is imposed on the density of the conductive pattern and that difficulties are encountered on bringing the mask pattern in register with the through-holes and on manufacturing the mask.
A printed wiring board comprising a metal core is disclosed by Donald Dinella in U.S. Pat. No. 3,296,099 and in two technical publications, The Western Electric Engineer, July 1965 issue, pages 24-29, and July 1973 issue, pages 18-25. A metal-core printed wiring board is preferred to a printed wiring board comprising a solid insulator substrate because the former is dimentionally more stable, mechanically stronger, more heat-dissipating, and yet less expensive. When made of a magnetic material, such as mild steel, the metal core serves as a magnetic shield and provides a magnetic circuit. Through-holes of a metal-core printed wiring board have funnel-shaped or outwardly flaring hole-end surfaces. The hole-end diameter of each through-hole is longer than the diameter of an inner principal portion of the through-hole. Insofar as the present applicants are aware of, the pattern edges or borders of a metal-core printed wiring board have also been extended to the through-hole surrounding portions. As a result, a conventional metal-core printed wiring board has been defective as regards the less dense conductive pattern and the above-mentioned difficulties.